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 A283361 a(n) = Product_{k=2..floor(2*n/3} (k^2 (mod 2n-1)). 1
 0, 0, 1, 4, 4, 0, 4, 4, 0, 2, 4, 0, 1, 0, 0, 1, 4, 0, 0, 12, 0, 20, 9, 0, 36, 0, 0, 29, 0, 0, 27, 13, 0, 0, 65, 0, 40, 67, 0, 0, 26, 0, 64, 0, 0, 71, 0, 0, 0, 53, 0, 49, 81, 0, 33, 83, 0, 98, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS If a(n) > 0 then 2n - 1 is prime. Proof: If 2n-1 is composite number, then the greatest prime factor (Gpf) of 2n-1 be 3 <= Gpf(2n-1) <= floor(2*n/3) and a(n)=Product_{k=2..floor(2*n/3)} (k^2 (mod 2n-1)) = 0. Else Gpf(2n-1) = 2n-1. a(n)>0 2n-1 is prime number. - Zhandos Mambetaliyev, Mar 06 2017 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 0..10000 MATHEMATICA Table[PowerMod[Product[k, {k, 2, Floor[2 n/3]}], 2, 2 n - 1], {n, 0, 61}] (* Michael De Vlieger, Mar 06 2017 *) PROG (PARI) a(n)=prod(k=2, 2*n\3, k^2)%(2*n-1) \\ Charles R Greathouse IV, Mar 06 2017 (PARI) a(n)=lift(Mod((2*n\3)!, 2*n-1)^2) \\ Charles R Greathouse IV, Mar 06 2017 CROSSREFS Cf. A004523 (Two even followed by one odd). Sequence in context: A158100 A104287 A174611 * A138518 A290799 A155836 Adjacent sequences:  A283358 A283359 A283360 * A283362 A283363 A283364 KEYWORD nonn,easy AUTHOR Zhandos Mambetaliyev and Michel Marcus, Mar 05 2017 EXTENSIONS More terms from Charles R Greathouse IV, Mar 06 2017 STATUS approved

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Last modified March 18 11:15 EDT 2019. Contains 321283 sequences. (Running on oeis4.)